It is known to provide agricultural tractors with a mechanical draft and position sensing system for controlling a hitch to which implements may be attached, such as described in U.S. Pat. Nos. 4,817,726 and 4,091,877. Most low horsepower tractors use top link draft force sensing because lower link draft sensing is difficult to implement on a low horsepower tractor due to space constraints. But, top link draft force sensing does not work well under high draft loads with certain types of implements.
Some current production John Deere tractors, such as all Series Model 5000 and 03 and 05 Series Model 6000 tractors, have a mechanical hitch or rockshaft control unit which has levers or cams which operate the spool of a hitch control valve. Such units have been difficult to assemble since many small parts must be mounted on sleeves or shafts on the inside of a rockshaft cylinder housing. Such production systems also have required a complicated set of linkages to transmit sensed draft force from the upper draft link to the hitch control unit.